Previous studies in our laboratory have demonstrated that infection of susceptible strains of mice with Friend leukemia virus leads to a suppression of lymphocyte mediated cytotoxicity, an in vitro correlate of cell-mediated immunity. These studies will be continued to further characterize this suppression. The relative roles of the spleen focus forming virus component and the lymphatic leukemia virus component of the Friend virus complex in this suppresive event will be assessed. Studies will also be performed to determine the target cell of the virus which is responsible for the marked suppression. The mechanism by which the cytotoxicity of effector cells is suppressed will be studied by means of antiserum and enzymatic treatment of cytoxic splenic lymphocytes from both control and FLV infected alloantigen sensitized mice. Studies will also be undertaken to suppress in vitro the cytotoxic activity of effector lymphocytes from normal alloantigen sensitized mice. Continued studies of this nature provide insight into mechanisms by which oncogenic agents can alter host defense mechanisms and outcome of the transformed cell-host interaction.